First round (group stage): The 52 teams were divided into nine groups (seven groups of six teams and two groups of five teams) to play home-and-away round-robin matches. The winners of each group will qualify for the 2018 FIFA World Cup, and the eight best runners-up will advance to the second round (play-offs).
Second round (play-offs): The eight best runners-up from the first round will play home-and-away over two legs. The four winners will qualify for the 2018 FIFA World Cup.
https://en.wikipedia.org/wiki/2018_F...ification#UEFA